1. Field of the Invention
The present invention relates to metallized film capacitors, and, particularly, to those for AC application.
2. Description of the Prior Art
Conventional metallized film capacitors, which are structured simply by rolling long metallized film into a cylindrical or flat shape, may emit smoke or be burnt when they are subjected to such a lethal dielectric breakdown that is exceeding the limit of the self-healing function inherent to the metallized film capacitor. In order to provide the capacitor with an additional protection function, that results in only slight decrease in its static capacitance but prevents the emitting of smoke or burning from actually occurring, an improvement had previously been proposed and filed as a patent application by the present inventors, Japanese Patent Application Nos. Sho 56-41180, Sho 56-82265 and Sho 56-95856 (U.S. Ser. No. 225,440, European Patent Application No. 81100315.1, Indonesian Patent Application No. 7859).
The proposed improvement comprises splitting the vapour-deposited metal electrode into a plurality of segments to give them a feature of severing a particular electrode from the group of other segmental electrodes by partly fusing off, when a breakdown occurs locally at the particular segmental electrode.
This structure of simply splitting the metallized film electrode into a plurality of segments has, however, a number of unsolved problems in the aspects of the reliability in its protection function and of the loss in material in its manufacturing process.
The structure of a wound-type metallized film capacitor is shown in a perspective view of FIG. 1, while that of a stacked-type is shown in a perspective view of FIG. 2. In FIG. 1, a vapour-deposited metal layer 2 on a dielectric material film 1 is splitted into segmental electrodes by grooves 3 that extend in the width-wise direction of the metallized film or strip such as a metallized polymeric material sheet or metallized paper. In FIG. 2, the stacked structure of the capacitor automatically means the splitting of the vapour-deposited metal layer 2 in FIG. 1(b) into the segmental electrodes.
In both of the wound-type and stacked-type capacitor comprising the segmental electrodes, the shown structure equals the aggregate of a multiplicity of minute capacitors connected with external connecting members represented by sprayed-metal layers. In a metallized film capacitor structured as shown in the above, when an excessively large current is flown through a particular segmental electrode as a result of the dielectric breakdown of the film and the current density in the segmental electrode participated in the dielectric breakdown rises abruptly, a part of the vapour-deposited metal electrode is fused off along a line or in a belt-like shape to electrically disconnect the segmental electrode from the others in the group of the segments. A sectional view of FIG. 3 illustrates a state of a capacitor element when such the phenomena occurs, namely, a dielectric breakdown results between the confronting pair of electrodes at a point 4. At least one of the segmental vapour-deposited electrodes 2a and 2b participated in the dielectric breakdown causes the belt-like fusing-off 5 of the metal layer to disconnect the segment from the body of the capacitor. In FIG. 3, numeral 6 represents layers of sprayed metal of the external connecting members.
FIG. 3' is a similar sectional view of a capacitor wherein one of both-side metallized films 1 is sandwiched between two non-metallized dielectric material films 18.
The both-side metallized films 1 have similar segmental vapour-deposited electrode layers 2a and 2b, and perform similar disconnecting function as described in connection with the films shown in FIG. 3. In both of FIGS. 3 and 3', the films are dipicted as they are loosely stacked with substantial spaces therebetween, though they are actually closely contacted with each other. Such a phenomenon likewise occurs in the cases wherein the dielectric breakdown consecutively develops to the adjacent layers.
The metallized film capacitor of the segmental electrode-type has thus been improved in its protection function that can prevent the lethal smoke-emitting or burning failure from actually occurring but may lead to a slight decrease in its static capacitance, even if the capacitor is subjected to an unexpected condition wherein an abnormally high voltage is applied to it or it is brought under an abnormally high temperature condition to cause a dielectric breakdown.
Accordingly, the metallized film capacitor of the segmental-electrode type does not need to be provided with a special protection device because it itself has the above-indicated improved self-protecting function, and can be made smaller in size and less expensive in corresponding degree, as compared with the conventional ones. In contrast to this, the latter comprising a single continuous vapour-deposited metal electrode needs to be provided with the special protection device for an unexpected service condition thus making the capacitor large in size and expensive in its manufacturing cost.
Although the above-indicated protection function can essentially be derived from the structure of the capacitor embodied as shown in FIG. 1 or FIG. 2, there still remains a number of unsolved problems. The problems may be itemized as follows; (1) the reliability of the protection function is unsatisfactorily low, (2) there is a considerable restriction on the width W (the width of the segmental electrode in the case of wound-type capacitor illustrated in FIG. 1 or the cutting width in the case of stacked-type capacitor illustrated in FIG. 2), and, as a result, the loss in material is large at the splitting groove (3 of FIG. 1) or in the cutting operation, and (3) disconnecting performance of the segmental electrode largely depends upon the condition of the part wherein the vapour-deposited metal electrode contacts the sprayed metal layer, and therefore the disconnecting characteristics largely deviate among the respective segmental electrodes.